p6-Type-Tiny.pm6

use nqp;

my package EXPORTHOW {
    package SUPERSEDE {
        class subset is Metamodel::SubsetHOW {
            has @!ss-params;
            method parameterize($obj,*@params) {
                @!ss-params = @params;
                $obj;
            }

            method accepts_type($obj,$checkee) {
                nqp::p6bool(
                    nqp::istype($checkee, $.refinee($obj)) &&
                    nqp::istrue($.refinement($obj).($checkee,|@!ss-params))
                )
            }
        }
    }
}

perl6 -Ilib -e '
  use Type::Tiny; 
  subset IntsGreaterThan of Int where { $^a > $^b }; 
  say 1 ~~ IntsGreaterThan[2]; say 3 ~~ IntsGreaterThan[2]'